Kinetic transitions during non-partitioned ferrite growth in Fe-C-X alloys

Ferrite growth behavior in Fe-C-Mn alloys has been studied using controlled decarburization experiments. The recently reported transition from LENP (local equilibrium-negligible partitioning) kinetics, at lower temperatures, to PE (paraequilibrium) kinetics, at higher temperatures, is shown to behave self-consistently over a range of Mn contents and temperatures and long-lived intermediate states between LENP and PE persist over a well-defined range of temperature and composition. A simple model which quantitatively describes the experimental observations over a range of composition and temperature is proposed. A key feature of this model is the introduction of an alloying element capacity of the moving α/γ interface, XMn*. The introduction of this quantity is purely guided by the experimental data and, at present, there is no physically based method for calculating it. Once XMn* is defined, multiple-jump kinetic analysis quantitatively describes the experimental observations over an impressive range of growth behaviors.